The importance of determining biological volume changes such as changes in fluid content of biological volumes has been recognized and reported, for example, by M. Pomerantz et al, in "Annals of Surgery," Vol. 171, No. 5, May 1970, at pages 686-691; by I. R. Berman et al, in "Archives of Surgery," Vol. 102, Jan. 1971, at pages 61-64; by J. N. Van deWater et al, in "Chest," Vol. 64 of Nov. 1973, at pages 597-603; and by R. V. Luepker et al in "American Heart Journal," Vol. 85, No. 1, Jan. 1973, at pages 83-93.
It has further been found, as reported in these publications, that the electrical impedance of biological volumes is correlated closely with the fluid content of the biological volumes.
In a conventional technique for measuring fluid volume, for example, intrathoracic fluid volume, the trans-thoracic electrical impedance is measured by connecting a constant current oscillator, of a frequeny of, for example, 100 kHz, between the neck and the lower abdomen of a patient. Voltage pickup electrodes are connected to the base of the neck of the patient and to a region slightly below the xiphisternal joint of the patient. The voltage at the pickup electrodes is applied through a receiver including an amplifier and a detector. The detected voltage output of the receiver is applied to a display device, such as a strip chart recorder or a digital indicator. The display device thus indicates the eletrical impedance between the pickup electrodes.
The equipment which may be employed for this type of measurement is disclosed, for example, in U.S. Pat. No. 3,340,867 to Kubicek, and U.S. Pat. No. 3,874,368 to Asrican.
The importance of measuring the trans-thoracic electrical impedance is apparent from the reports, which indicate that this technique may be applied to detect the early occurrence of fluid--intra or extra pulmonary--which is not easily found by auscultation or x-ray techniques. Thus, the above Pomerantz et al publication reports changes in the electrical impedance as much as 45 minutes prior to detectable alterations in central venous pressure, pulmonary compliance, arterial pressure, or changes in blood gases. The Van deWater publication further reports detectable drops in impedance resulting from over-hydration that were too subtle to be appreciated clinically or by x-ray examination.
In measurement of the electrical impedance of biological volumes by the above technique, intrathoracic gas and biological fluids are the main variable components. As a result, the impedance measured varies cyclically with the cardiac output, as well as with respiration. In addition, variation is noted due to movement of the subject. Such cyclic variation is shown, for example, in the above-mentioned U.S. Pat. No. 3,874,368. In view of this cyclic variation of the electrical impedance, various techniques have been employed in the past for determining overall changes in the electrical impedance that could be correlated with variation in the tissue fluid content. For example, as reported in the above publication of Van deWater et al, only the lowest readings are recorded, which occur at the end of expiration and correspond to the functional residual capacity level, so that respiratory variations which vary from 1.0 to 2.0 ohms with a vital capacity maneuver are avoided. In general, the observations of variation in electrical impedance for this purpose have required "eye-balling" of the recorder chart and manual averaging and plotting techniques. It was not possible in prior equipment to ascertain edema with the digital indicator, nor was it possible to obtain an automatic printout of values suitable for reliable indication of variation in tissue fluid content.
The present invention is therefore directed to the provision of means for readily indicating changes as fluid content changes of biological volumes, whereby the observer is not required to guess or estimate the values, nor to employ only certain portions of a cyclically varying electrical impedance measurements for ascertaining fluid changes.
Briefly stated, in accordance with the invention, a non-invasive monitor, such as a fluid monitor, is provided for overcoming the above problems, wherein means are provided for electronically averaging detected signals corresponding to the instantaneous electrical impedance of the biological volume under investigation. The time constant of the averaging circuit is preferably in the range of 2 seconds to 15 minutes, so that variations due to cardiac output, respiration and movement do not interfere with the measurement. As a consequence, the recorder provides a recording which is related directly to the fluid content of the tissue under investigation without the necessity for mental averaging or plotting, whereby inexperienced clinical personnel may be able to readily interpret the recording. In addition, due to the averaging of the signals, it is possible to employ a digital readout corresponding to fluid volume.
In a particularly advantageous environment of the invention means are provided for varying the time constant of the circuit so that it may be adapted to any desired conditions and in order to enable use of the equipment for other purposes, the variation of time constant may fall within the range of about 0 to 15 seconds.
In a simple environment of the invention, the time constant may be controlled as a function of the capacity of a capacitor in an amplifier in the circuit, whereby variation of the time constant of the circuit may be varied by varying the capacity of the capacitor.
The term "time constant" is employed in its specific sense, as a measure of a transient condition in a circuit having a resistance and a reactance. While a circuit in accordance with the invention may employ a resistance and a capacitance in the time constant circuit, the term is employed herein in its more general sense as a matter of the time that it takes a varying value to obtain 63% of its ultimate value once starting at an initial value, assuming no variation in the conditions causing the change of the value.